Method to determine light scattering efficiency of pigments

ABSTRACT

A method for determining light scattering efficiency and a method to determine undertone of pigments or mixtures containing white pigments are described. The methods are based on measuring total transmittance of pigment slurries at a wavelength of at least 600 nm. Transmittance measurements can be used to calculate light scattering efficiencies and undertone which correlate with performance in end-use applications.

This application is a continuation-in-part of a application Ser. No. 09/240,251 filed on Jan. 29, 1999 now granted, U.S. Pat. No. 6,040,913, which is a continuation of Ser. No. 08/707,693 filed on Sep. 4, 1996, now U.S. Pat. No. 5,902,441.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a method of measuring light scattering efficiency of white pigments and mixtures containing at least one light scattering component.

Light scattering properties are the primary product attributes of white pigments, particularly TiO₂ pigments. Precise, accurate evaluations of light scattering properties of pigments are important to guide product development, improve control of the manufacturing process, and aid consumers as they optimize use of pigments.

Two key properties in essentially all end-use applications of white pigments are (a) scattering efficiency for delivering opacity, which relates to hiding power and tint strength; and (b) undertone, which relates to the change of scattering efficiency as the wavelength of light varies. Opacity is the ability of a pigment to mask the features of the underlying substrate. Undertone of a white pigment may be described as the hue (bluish or reddish) of a gray paint containing the pigment in an admixture with a standard black pigment. Generally, a bluish undertone is characteristic of small particle size, while a reddish undertone is characteristic of larger and less uniform particle size. Undertone is particularly important in applications where color matching is needed.

Light scattering properties are typically measured on end-use products or lab simulations of end-use applications. Such measurement procedures are often complex, time-consuming and subject to errors due to variability in raw materials, variability introduced by having a multi-step process, and operator variability. Therefore, product characterization and process control have generally focused on fundamental properties, such as particle size measurements. Correlations between particle size and end-use properties are often used to infer actual optical properties of the pigments. Most particle size analyzers report particle sizes based on theoretical equations that assume a spherical shape. Therefore, the reported sizes are equivalent spherical diameters. However, pigments can have complex microstructures. For example, TiO₂ pigments contain twinned and aggregated primary crystals and non-spherical particles. Further, most commercial grades of TiO₂ pigments have hydrous oxide coatings. Therefore, measurements of equivalent spherical diameters may not accurately predict end-use performance of a pigment.

Pigments provide opacity or hiding power by diffuse light scattering in end-use applications. Measurements of opacity that do not include the diffuse component of transmittance do not provide results that reliably correlate with end-use properties.

Heretofore, scattering efficiency has typically been measured at a green light wavelength (540-580 nm) in the center of the visible spectrum where the eye is most sensitive. Undertone characterizes the change of scattering efficiency across the visible spectrum and therefore has been determined by comparing scattering efficiency at a blue wavelength relative to scattering efficiency at a red wavelength of light. Test methods in the prior art characterize pigments by (1) measurements on the end-use or a lab simulation of the end-use application, or (2) measurements on slurries of the pigment.

As an example of the first type, Bruehlman and Ross in J. Paint Techn. 1969, vol. 41, pp. 584-596 disclose use of transmission measurements using a spectrophotometer with an integrating sphere to determine scattering efficiency. Measurements are taken at 5500 Å (550 nm) of TiO₂ coating formulations deposited on plastic sheets. An improvement to the method of Bruehlman and Ross was described by Abrams, et al., in “Proceedings of 1996 TAPPI Coating Conference”, TAPPI Press, Atlanta, Ga., 1996, pp. 185-192. For applications where TiO₂ is used in porous media, e.g., paper coatings or paint films formulated above the critical pigment volume concentration, Abrams, et al. incorporate information on the porosity of the coating into calculations of hiding power.

As an example of the second type, Losoi in “Proceedings of 5^(th) International Paint Congress”, Sao Paulo, Brazil, Sep. 15, 1997, p. 213 (ABRAFATI, Brazilian Association of Paint Manufacturers, Rua Augusta, 2516-2° andar -cj.22-01412-100-Sao Paulo-SP) describes methods for determining optical properties of TiO₂ pigments dispersed in water. These pigments are aggregates of smaller primary crystals. Absorbance of a dilute slurry at 550 nm is used as a predictor of gloss properties. It also gives some indication of hiding power or tint strength, but correlations are limited unless independent information is available on the primary crystal size. A method for measuring undertone is based on the ratio of absorbance measurements of a dilute slurry of TiO₂ at 400 and 700 nm.

Turnstall, U.S. Pat. No. 4,279,512 discloses a method for measuring mean particle size of particles suspended in a continuous medium which involves measuring non-scattered radiation which passes through the continuous medium at three different wavelengths, R1, R2 and R3. Attenuation ratios are determined from the measurements. The attenuation ratios are compared with a set of previously calculated attenuation ratios for a selection of mean particle sizes, corresponding to the determined attenuation ratios. Use of the method to measure mean particle size of TiO₂ particles in aqueous suspension is disclosed.

Methods based on characterization of pigment slurries have major advantages over methods that require preparation of the end-use product: they are faster, less expensive, and can normally be determined with much higher precision. But prior art methods using pigment slurries have had limitations that restrict their ability to give results which directly correlate with performance in end-use applications.

Scattering performance is very sensitive to the difference between the refractive index of the pigment particle and the refractive index of the medium around the particle. The refractive index of water is low compared to the refractive index of the medium in end-use applications. Therefore measurements of optical properties of aqueous slurries of pigments in the blue-green region of the visible spectrum (450-600 nm) may provide inaccurate information regarding performance in an end-use medium.

Recently, E. Tan of Rhopoint Instrumentations (located in the UK), published information on a wet film hiding power meter in the supplement section of the March, 1999 issue of Product Finishing (pp. S2-S3). This device measures the opacity (ratio of the black over white reflectance of a coating) as a function of film thickness. The devise offers automation and computer data analysis but relies on well-known measurements and methods of analysis.

SUMMARY OF THE INVENTION

This invention provides a method to determine pigment properties that correlate with performance of a pigment in an end-use application where the pigment is dispersed, for example, in a paint or coating. This invention provides a rapid, precise method that directly provides light scattering efficiency of a pigment, which comprises:

(a) preparing a dilute slurry of a pigment in a continuous liquid medium having a known concentration (C), where concentration is the concentration of pigment in the slurry;

(b) measuring total transmittance (T) of the slurry at a wavelength (λ) of at least 600 nm;

(c) converting the transmittance measurement (T) to optical density (OD) by the equation

OD=−log (T)

and

(d) dividing the optical density by the concentration (C) of the pigment in the slurry to determine light scattering efficiency of the pigment.

In particular, the method of this invention can be used to determine light scattering efficiency of a number of white pigments. Preferably the pigment is selected from the group consisting of both rutile and anatase titanium dioxide, zinc oxide, silica, calcium carbonate, calcium sulfate and clay, and mixtures thereof. More preferably, the pigment is rutile or anatase titanium dioxide (TiO₂).

This invention further provides a method to determine pigment undertone, which comprises:

(a) preparing a dilute slurry of pigment particles in a continuous liquid medium having a known concentration (C), where concentration is the concentration of pigment in the slurry;

(b) measuring the total transmittance (T₁) of the slurry at a first wavelength, λ₁, between 500 and 600 nm;

(c) measuring the total transmittance (T₂) of the slurry at a second wavelength, λ₂, between 600 and 1000 nm;

(d) converting the transmittance measurements to optical densities (OD) by the equation

OD=−log (T)

(e) dividing the optical densities by the concentration (C) of the pigment in the slurry to calculate OD/C;

(f) determining undertone of the pigment by the degree of change in OD/C between λ₁ and λ₂.

A number of equations can be used to determine undertone, or the change in OD/C between the two wavelengths, λ₁ and λ₂. Preferably, undertone is determined according to the equation: ${undertone} = {- {\frac{\left\lbrack {\ln \frac{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}}{{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right\rbrack}{\left\lbrack {\ln \frac{\lambda_{1}}{\lambda_{2}}} \right\rbrack}.}}$

OD₁/C at λ₁ is the optical density calculated from the transmittance measurement at wavelength λ₁ divided by the concentration of pigment in the slurry. OD₂/C at λ₂ is the optical density calculated from the transmittance measurement at wavelength λ₂ divided by the concentration of pigment in the slurry.

The present invention also provides a method to determine light scattering efficiency of a mixture containing at least one light scattering component comprising:

(a) preparing a dilute slurry of the mixture in a continuous liquid medium having a known concentration, where concentration is the concentration of mixture solids in the slurry;

(b) measuring the total transmittance (T) of the slurry at a wavelength (λ) of at least 600 nm;

(c) converting the transmittance measurement (T) to optical density (OD) by the equation

OD=−log (T);

and

(d) dividing the optical density by the concentration of mixture solids to determine light scattering efficiency of the mixture.

The continuous medium may be selected from the group consisting of water, organic solvents, and paint resins. The present invention is useful for mixtures where the light scattering component is a white pigment selected from the group consisting of rutile titanium dioxide, anatase titanium dioxide, zinc oxide, silica, calcium carbonate, calcium sulfate and clay, and mixtures thereof. In addition to these white pigments, light scattering component includes plastic pigment, latex polymer particles and other materials which are not completely soluble in the water (or solvent) and that provide some measurable light scattering.

The present invention also provides a method to determine undertone of a mixture containing at least one light scattering component, comprising:

(a) preparing a dilute slurry of the mixture in a continuous medium having a known concentration (C), where concentration is the concentration of the mixture solids in the slurry;

(b) measuring total transmittance (T₁) of the slurry at a wavelength, λ₁, between 500 and 650 nm;

(c) measuring total transmittance (T₂) of the slurry at a wavelength, λ₂, between 650 and 1000 nm;

(d) converting the transmittance measurements to optical densities (OD) by the equation

OD=−log (T)

(e) dividing the optical densities by the concentration (C) of mixture solids in the slurry to calculate OD/C;

(f) determining the undertone of the mixture by the degree of change in OD/C between λ₁ and λ₂.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a graph showing relative optical densities for a series of pigments and the corresponding relative scattering coefficients (S) for these pigments in a paper coating.

FIG. 2 is a graph showing correlation of relative optical densities with scattering coefficients in a paper coating and the related regression R² values.

FIG. 3 is a graphical illustration for characterizing the light scattering of a mixture containing at least one light scattering component. In the Figure the light scattering efficiency of the mixture calculated according to Equations 1 and 2, below, is compared to the undertone of the mixture calculated according to Equation 3 or 4, below.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods to determine light scattering properties of white pigments. Pigments are commonly dispersed in films or coatings in an end-use. The methods of this invention provide a rapid process to determine optical density and undertone of a pigment with the pigment dispersed in a continuous liquid medium. These methods are particularly useful to provide information that correlates to performance of the pigment in end-use applications.

Instrumentation

In this invention, a spectrophotometer is used to measure transmittance (T) through a sample of a dilute slurry of a white pigment. Optical density is calculated as OD=−log(T). While there are numerous spectrophotometers available commercially, for purposes of this invention, it is critical that the spectrophotometer provides a measure of the total transmittance of the sample. “Total transmittance” includes both regular (rectilinear or straight-through) and diffuse transmittance. Regular transmittance relates to the energy that passes directly through the sample. Diffuse transmittance relates to energy that is scattered as it passes through the sample.

Total transmittance is important in a measurement of a white pigment to correlate the results with end-use performance, since performance of a pigment in the end-use depends on diffuse scattering. For correlations of measurements with end-use, e. g., in a paper coating or a paint film, use of total transmittance at the appropriate wavelength can provide a 1:1 relationship between optical density and the light scattering efficiency (hiding power) in the end-use application. If the instrument measures only regular transmittance, some differences between pigments can be characterized, but there is no 1:1 correlation between optical density and end-use performance.

One method of obtaining measurements of total transmittance of a sample is to equip the spectrophotometer with an integrating sphere. Such a modification is available commercially. Other methods can be used to obtain measurements of total transmittance.

Sample Preparation

In the method of this invention, a dilute slurry of pigment particles in a continuous liquid medium of a known concentration is prepared. Concentration is referred to herein as the concentration of pigment in the slurry. The preferred pigments to be used in the method of this invention are commercial white pigments commonly used as opacifiers. These pigments include rutile titanium dioxide (TiO₂), anatase TiO₂, zinc oxide, silica, calcium carbonate, calcium sulfate, zinc sulfide, basic lead sulfate, basic lead carbonate, antimony oxide, lithopone or the like, and clays, e. g., kaolin clay, and mixtures thereof. Preferably the pigment is selected from the group consisting of both rutile and anatase titanium dioxide, zinc oxide, silica, calcium carbonate, calcium sulfate and clay, and mixtures thereof. Most preferably, the pigment is rutile or anatase titanium dioxide.

Typically and preferably, the continuous medium is water, however, other continuous media that can be used include other hydrophilic media or hydrophobic media such as organic solvents, or a paint resin.

The concentration of the pigment in the dilute slurry will depend on the path length of the spectrophotometer cell. The pigment concentration should be within a range of concentration where optical density is directly proportional to concentration. The concentration should also be in a sensitive range for the instrument where there is a high signal-to-noise ratio. For example, for a slurry of TiO₂ in water, using a cell having a path length of 1 cm, the concentration of TiO₂ in the slurry should be in the range of 5 to 50 ppm, preferably 10-30 ppm. For cells having a path length less than 1 cm, a higher concentration of TiO₂ in the slurry may be tolerated, while maintaining the linear relationship of optical density to TiO₂ concentration. Conversely, for cells having a path length greater than 1 cm, lower concentrations of TiO₂ in the slurry should be used.

For other pigments and for continuous media other than water, concentration may need to be higher or lower to satisfy the proportional relationship between optical density and concentration and also high signal-to-noise. For low refractive index pigments, such as calcium carbonate and clays, concentrations on the order of 100-300 ppm, are preferred when using water as the continuous medium. It should be recognized that one skilled in the art will be able to determine the optimum pigment concentration to use.

A dilute pigment slurry is prepared by mixing a slurry of the pigment or solid pigment with a sufficient amount of the continuous medium to provide the desired concentration. Optionally, the slurry is subjected to a pretreatment prior to dilution. Examples of pretreatment methods include grinding and sonication. Sonication is a preferred step when preparing a slurry from a dry sample. Preferably the dilute slurry will be prepared by stepwise dilution of pigment slurries of higher concentration. It is important to accurately prepare the dilute slurry sample as errors in concentration will negatively impact the accuracy of calculated results and affect the ability to correlate results with end-use performance of the pigment.

Optionally, a dispersant is added to the dilute slurry. Dispersants include polyacrylates, citric acid, boric acid, acetic acid, adipic acid, succinic acid, tetrapotassium pyrophosphate(TKPP), ethylenediamine-tetraacetic acid, triethanol amine (TEA), 2-amino-2-methyl-1-propanol (AMP), trimethylol propane (TMP), or the like, and mixtures thereof.

Measurements

The dilute pigment slurry should be added to a spectrophotometer cell per instrument instructions to provide a sample.

The method of this invention can be used to determine optical properties of a white pigment, which correlate with end-use performance by measuring transmittance of a slurry containing the pigment at a wavelength of at least 600 nm, and typically from 600 to 1000 nm. The preferred wavelength for characterizing light scattering efficiency or hiding power is shifted from the green region of the spectrum (540 to 580 nm) to a higher wavelength. The shift depends on the end-use application for which the pigment is intended and the refractive index of the continuous medium in which the transmittance measurement is taken. When optical density is based on a transmittance measurement taken in water (refractive index 1.33), the preferred wavelength to use for a pigment for use in a pigment-filled paper is shifted by about 50 to 100 nm to about 600 to 700 nm. For a pigment to be used in a paper coating, when the continuous medium is water, the preferred wavelength is shifted by about 150 nm to about 650 to 750 nm, and more preferably, the wavelength is 700 nm. For a pigment to be used in a plastics application, especially when the pigment is TiO₂ and the TiO₂ concentration is less than 15%, the preferred wavelength to determine optical properties is also in the range 650 to 750 nm, when the continuous medium is water.

For a pigment to be used in coatings applications, such as in a paint, the preferred wavelength from which to determine optical properties is 750 to 1000 nm, when the continuous medium is water. This range is broader than the range for paper and plastics applications since there is greater variability in coatings formulations. The formulation may be water-based or solvent-based. Pigment volume concentration (PVC), and particularly the TiO₂ volume concentration, will also impact the preferred wavelength. By “TiO₂ volume concentration”, it is meant the ratio, by volume, of TiO₂ pigment to the remaining components of the formulation and may be expressed by the following formula: ${\% \quad {TiO}_{2}\quad {Pigment}} = {\frac{{volume}\quad {of}\quad {TiO}_{2}\quad {Pigment}}{{Total}\quad {volume}\quad {of}\quad {coating}\quad {solids}} \times 100}$

For a typical latex paint having a TiO₂ volume concentration of about 20%, the preferred wavelength to be used in the method of this invention is from 750 to 850 nm. For a latex paint having a TiO₂ volume concentration of about 30%, the preferred wavelength for measurement is 850 to 1000 nm.

Total transmittance of the sample is measured at the determined wavelength for the purpose of determining light scattering efficiency. The measured value of total transmittance is used to calculate optical density and subsequently scattering efficiency of the pigment, as described below.

Undertone in an end-use application is determined by comparing light scattering at the lower end of the visible spectrum (λ₁=blue wavelength) to scattering at the upper end of the visible spectrum (λ₂=red wavelength). To determine undertone in dilute pigment slurries, these wavelengths both need to be shifted higher, just as the wavelength used to characterize optical density or opacification performance is shifted higher. Instead of a blue wavelength, total transmittance of the sample is measured using a first wavelength, λ₁, in the range of 500 to 650 nm and instead of red light, total transmittance is measured using a second wavelength, λ₂, in the range of 650 to 1000 nm. Optical densities are calculated at each wavelength from the measurements of total transmittance. The optical densities are used to calculate the undertone of the pigment, as described below. Optical density typically decreases as the measuring wavelength increases. Pigments having curves of optical density versus wavelength with steeper declines as wavelength increases correspond to those having higher undertone values.

Optionally a “blank” run is made prior to measuring the total transmittance of the dilute pigment slurry sample to calibrate the instrument at each wavelength. To run a “blank”, a spectrophotometer cell should be filled with the same continuous liquid medium, i. e., water, solvent or resin, used to prepare the dilute pigment slurry.

Surprisingly, the combination of measuring total transmittance, which includes both regular and diffuse transmittance, and taking the measurement at a wavelength greater than 600 nm, provides excellent correlation between optical density and the opacification performance, i. e., scattering efficiency in an end-use application.

Calculations

Optical density (OD) should be calculated from total transmittance (T) at each wavelength based on equation 1.

OD =−log(T)  (1)

Concentration of the pigment in the slurry sample, C, in ppm, is known from sample preparation. By dividing by the concentration, light scattering efficiency can be calculated using equation 2.

light scattering efficiency=10⁶ ×OD/C  (2)

For convenience and reliability, the optical density or light scattering efficiency can be reported relative to a standard pigment having a known value. In this way the optical properties of a standard pigment can be used for comparison with other sample pigments.

Undertone is calculated from the light scattering efficiencies calculated as described above at two wavelengths, λ₁ and λ₂. Many relationships can be used to characterize the rate of change of optical density with wavelength, but the logarithmic relationship of equation 3 is preferred: $\begin{matrix} {{undertone} = {- \frac{\left\lbrack {\ln \frac{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}}{{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right\rbrack}{\left\lbrack {\ln \frac{\lambda_{1}}{\lambda_{2}}} \right\rbrack}}} & (3) \end{matrix}$

OD₁/C at λ₁ is the optical density calculated from the transmittance measurement at wavelength λ₁ divided by pigment concentration in the slurry. OD₂/C at λ₂ is the optical density calculated from the transmittance measurement at wavelength λ₂, divided by pigment concentration in the slurry. For convenience, a scale factor can be included, if desired.

An alternative to equation 3 to provide undertone is provided in equation 4. $\begin{matrix} {{undertone} = {- \frac{\left( {{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}} - {{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right)}{\lambda_{1} - \lambda_{2}}}} & (4) \end{matrix}$

While calculated undertone values determined by the optical density determinations of this method may vary from conventional carbon black undertone (CBU) values, the results are consistent with each other in predicting the relative undertones for a group of pigments. CBU is described by Schaumann in U.S. Pat. No. 2,488,439, the teachings of which are incorporated herein by reference. The method of this invention to determine undertone is more rapid that the method described by Schaumann, which involves preparing oil mulls of a sample and a standard pigment with carbon black and comparing with a standard set of values. It has been found that calculated undertone values vary linearly with CBU and undertone values determined from optical density can be converted to the CBU-scale using the regression constants for this linear relationship.

Correlation with End-Use Performance

Assessment of end-use performance is not always readily available to intermediates manufacturers, nor are manufacturers of the end product often willing to test a new product without strong indication/support that the new product will provide an advantage to their process and/or their product. Therefore tests which provide good correlations with end-use performance are invaluable in product development, process control in pigment manufacture, and introduction to the market.

Measurement of pigment properties that correspond with end-use performance can be of assistance to both pigment manufacturers and their customers to improve formulations. A test based on spectrophotometric measurements taken of paint films or paper coatings deposited on plastic sheets provides results that reliably correspond with end-use performance. Such a test is described by Abrams, et al., in “Proceedings of 1996 TAPPI Coating Conference”, described above. For example, in this test, improvements in scattering efficiency can be directly correlated with equal or improved optical performance at reduced levels of TiO₂. However this test is time consuming. Replication error is better than most tests on end-use products but is still greater than 2%. Nonetheless, given the success of the test described by Abrams, et al., this test can be used for comparison with results obtained by the test method of this invention to support correlation with end-use performance.

The method of this invention is an improvement on the method described by Abrams, et al., for determining hiding power or light scattering efficiency of pigments. In the Abrams method, a coating formulation containing a particular pigment is prepared and coated onto a polyester film. The hiding power of the pigment is determined by measuring transmittance and reflectance of the film at a wavelength of 550 nm, followed by calculations that consider porosity of the coating. Surprisingly it has been found that determination of optical density via total transmittance measurements of a white pigment dispersed in a continuous liquid medium, when the transmittance measurement is made at a wavelength greater than 600 nm, provides results which correlate 1:1 with results from the method of Abrams, et al. Therefore, the present invention provides a simple test with results that correlate with performance of the pigment in an end-use application. Replication error by the method of this invention is only 0.5% which is a substantial improvement over the 2%+ error of the method of Abrams, et al.

Industrial Applications

The methods of this invention to determine light scattering efficiency and undertone of white pigments are useful to both pigment manufacturers and consumers. For example, these methods can be used in screening of pigments for various end-uses, in product and process development of pigments, and in optimizing formulations containing pigments.

Screening of pigments to compare light scattering efficiency and undertone can find utility in comparison of pigments to determine relative properties. Pigments can be evaluated, for example, for a particular scattering efficiency or for a desired undertone. As the results correlate with end-use performance, pigment consumers may gain benefit to understand which pigments are preferred in their products and formulations.

Screening of pigments is also useful to consumers in optimizing their formulations, for example, based on opacity. Less of a pigment having a higher light scattering efficiency may be used in place of a pigment having a lower light scattering efficiency resulting in the same opacity at a lower cost of manufacture. Alternatively, higher opacity may be achieved by replacing a pigment with a pigment having a higher light scattering efficiency.

In the manufacture of titanium dioxide pigments, by either the sulfate or chloride process, the pigment products are in the form of agglomerates and aggregates of TiO₂ crystals. Undertone measurements according to the method of this invention can be used to optimize the size of the crystals. Optical density measurements according to the method of this invention can be used to optimize the degree of agglomeration. These methods can be used as tools for quality control in the pigment manufacturing process. In contrast, conventional quality control methods, such as those based on particle size, cannot differentiate effects of crystal size and degree of agglomeration.

Crystal size is generally determined in the calciner for sulfate process and in the oxidation step of the chloride process. Degree of agglomeration is affected by grinding and milling steps. Understanding and controlling crystal size and agglomeration can be useful in new product development and process improvements.

Undertone is particularly important where color matching is critical, such as in color mixing of paints, in automotive refinishing, and matching of colored materials from different sources. Therefore the methods of this invention can readily provide manufacturers of TiO₂ pigments with information relating to quality of their pigment products and further assist their customers with formulations.

Method Applied to Mixtures

The present invention may also be adapted to determine the light scattering efficiency and the undertone of a mixture containing at least one light scattering component. In this case, the mixture is diluted with a continuous liquid medium to form a slurry. The concentration used in the determinations of optical density and undertone is the concentration of mixture solids in the slurry.

A mixture, as the term is used in this application, is a combination of materials where at least one of the materials scatters light to some measurable extent. Mixtures according to the present invention may be as simple as a combination of water with a light scattering component such as white pigment, as defined above. A mixture according to the present invention may also be a complete formulation or a combination of a part of the complete formulation that constitutes a film forming composition. Such film forming compositions include paints, coatings, inks, masking materials, adhesives, sealers, caulks, primers, or the like and sheet structures including paper. For example, a mixture according to the present invention may be a combination of white pigment with a resin binder, dispersants and other additives, or the mixture may be the complete formulation including white pigment, fillers, resin binders, additives, dispersants, defoamers, solvents and such. The preferred mixture for use in the present method is the complete formulation. Although the mixture may include a color pigment or dye, it is recommended for optimal determination of light scattering efficiency that any color material such as a color pigment or dye that is a part of the complete formulation be excluded from the mixture tested.

The term mixture solids means the weight of material present in the dry film formed by the mixture. For example, if the mixture is a combination of scattering particles (white pigment, plastic pigment or filler) and binder, the mixture solids present in the slurry formed in step (a) of the method is the sum of the weight of scattering particles and the binder solids. If the mixture is a complete coating formulation, the mixture solids is the sum of all the component solids present in the slurry.

As in the pigment test method described above, the continuous liquid medium includes water, organic solvents, and paint resins. The term “white pigment” includes rutile titanium dioxide, anatase titanium dioxide, zinc oxide, silica, calcium carbonate, calcium sulfate, clay, and mixtures of these. The term “light scattering component” includes white pigment as defined above and in addition plastic pigment, latex polymer particles and other materials which are not completely soluble in the water (or solvent) and that provide some measurable light scattering.

Unlike the application of the present invention in the determination of the scattering efficiency to a slurry of white pigment, its application to a mixture may not have direct correlation to the optical properties of the dry mixture film. One can understand this possibility if one considers that complete formulations often contain fillers or resins that provide some contribution to scattering by the porosity they introduce into the dry film. The present method provides a rapid, reliable measurement of scattering efficiency from analysis of a dilute wet film. Although the method does not provide a direct measurement of dry film porosity or optics, conclusions drawn from the method apply to the dry film.

In the application of the present method to mixtures containing at least one light scattering component, concentration ranges described above for pigment measurements are also applicable to the concentration ranges for mixtures. Wavelengths recommended for particular end-uses when the method is applied to pigment slurries are also recommended for use in the determinations for mixtures.

According to the present method the light scattering efficiency of a mixture can be determined by

(a) preparing a dilute slurry of the mixture in a continuous liquid medium having a known concentration, where concentration is the concentration of mixture solids in the slurry;

(b) measuring the total transmittance (T) of the slurry at a wavelength (λ) of at least 600 nm;

(c) converting the transmittance measurement (T) to optical density (OD) by the equation

OD=−log (T);

and

(d) dividing the optical density by the concentration of mixture solids to determine light scattering efficiency of the mixture.

The undertone of a mixture containing white pigment, may be determined by

(a) preparing a dilute slurry of the mixture in a continuous medium having a known concentration (C), where concentration is the concentration of the mixture solids in the slurry;

(b) measuring total transmittance (T₁) of the slurry at a wavelength, λ₁, between 500 and 650 nm;

(c) measuring total transmittance (T₂) of the slurry at a wavelength, λ₂, between 650 and 1000 nm;

(d) converting the transmittance measurements to optical densities (OD) by the equation

OD=−log (T)

(e) dividing the optical densities by the concentration (C) of mixture solids in the slurry to calculate OD/C;

(f) determining the undertone of the mixture by the degree of change in OD/C between λ₁ and λ₂.

The degree of change in OD/C may be calculated according to the equation 3: $\begin{matrix} {{{undertone} = {- \frac{\left\lbrack {\ln \frac{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}}{{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right\rbrack}{\left\lbrack {\ln \frac{\lambda_{1}}{\lambda_{2}}} \right\rbrack}}},} & (3) \end{matrix}$

wherein OD₁/C at λ₁ is the optical density calculated from the transmittance measurement at wavelength λ₁, divided by the concentration of mixture solids in the slurry and OD₂/C at λ₂ is the optical density calculated from the transmittance measurement at wavelength λ₂, divided by the concentration of the mixture solids in the slurry; or according to the equation 4: $\begin{matrix} {{{undertone} = {- \frac{\left( {{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}} - {{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right)}{\lambda_{1} - \lambda_{2}}}},} & (4) \end{matrix}$

wherein OD₁/C at λ₁ is the optical density calculated from the transmittance measurement at wavelength λ₁, divided by the concentration of mixture solids in the slurry and OD₂/C at λ₂ is the optical density calculated from the transmittance measurement at wavelength λ₂, divided by the concentration of the mixture solids in the slurry.

Comparison of the light scattering efficiency of the mixture to the undertone of that mixture provides an excellent quality control or formulation development tool. This comparison provides a method for characterizing the light scattering of a mixture containing at least one light scattering component comprising comparing the of light scattering efficiency of the mixture according to Equations 1 and 2 to the undertone of the mixture according to Equation 3 or 4. FIG. 3 shows such a comparison.

Using this comparison method, the present invention may be used to determine the dispersion stability of a mixture containing at least one light scattering component. For example, by comparing the light scattering efficiency of the mixture, (1) after addition of an electrolyte, (2) after changing the ionic strength of the mixture, or (3) after the addition of an increased amount of dispersant, (4) the addition of a second dispersant or (5) substituting one dispersant for another; a formulator can predict benefits or problems associated with formulation or raw material changes. Example 3 illustrates a use of the present method applicable to quality control or formulation development. Example 4 illustrates a use of the present method in measurement of dispersion stability. Those skilled in this art will appreciate other ways in which the present method may be used.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 shows a graph of relative optical density over concentration (OD/C) versus relative scattering coefficient (S) in an end-use formulation for paper coatings for a series of titanium dioxide pigments, described in the Examples, Table 1. Relative optical densities were determined using the method of this invention. Relative S values for the same pigments were determined using the method described by Abrams, et al, wherein paint formulations were deposited on Mylar® polyester sheets. The line drawn in the graph is the ideal 1:1 correlation line. When optical density is measured at 700 nm, the data closely follow the 1:1 correlation line. When optical density is monitored at the conventional green wavelength (550 nm), the performance is frequently over-predicted.

FIG. 2 shows a graph of regression slope and regression R² versus wavelength for optical density determinations. This graph is useful to select preferred wavelengths to measure total transmittance for the method of this invention for a paper coating end-use application. Total transmittance is measured at a number of wavelengths. Results for relative optical density over concentration (OD/C) are compared with relative scattering coefficients (S), determined by the method of Abrams, et al. The graph shows the linear regression slope of relative OD/C versus relative S for a number of wavelengths and the corresponding regression R² values. As can be seen from the graph, there is a 1:1 correlation (slope=1) and the highest R² values at a wavelength of about 700 nm.

FIG. 2 provides information to select preferred wavelengths for correlating measurements with performance in paper coatings. Similar comparisons of relative optical density/concentration with scattering coefficients, S, can be made by those skilled in the art for other end-use applications to select preferred wavelengths for measurements to correlate with end-use performance. Other end-use applications include pigment-filled paper, plastics, and paints.

FIG. 3 illustrates a technique for characterizing the coating composition by plotting the optical density of the coating formulation as a function of the undertone measured for that coating composition. This particular type of plot may be thought of as a fingerprint for the coating or mixture containing at least one light scattering component. This fingerprint may be used to anticipate shifts in the scattering characteristics of the coating with the change in the amount or the nature of a component of the composition.

Fingerprint characterization of the composition of is an example of one of the uses of the present invention in formulating or in quality control.

EXAMPLES Example 1

To demonstrate the ability of the test method of this invention to provide results that correlate with end-use performance, measurements were taken of a series of commercial titanium dioxide pigments described in Table 1.

TABLE 1 Primary Aggregate Degree of Pigment Crystal Crystal Size, Size, Agglomeration Sample Form microns (a) microns (b) (c) A Anatase 0.140 0.68 4.3 B Rutile 0.213 0.48 2.3 C Rutile 0.172 0.40 2.3 D Rutile 0.205 0.39 1.9 E Rutile 0.195 0.31 1.6 (a) Primary crystal size is determined by electron microscopy. (b) Aggregate size is determined using a Horiba LA-900 laser scattering particle size analyzer (available from Horiba Instruments, Inc., Irvine, California). (c) Degree of agglomeration is the ratio of aggregate size to primary particle size.

Anatase pigment A was prepared by the sulfate process. Rutile pigments B-E were prepared by the chloride process. For the rutile pigments, differences in primary crystal size were achieved by varying nucleant as described by Lewis and Braun in U.S. Pat. No. 3,208,866 and Allen and Evers in U.S. Pat. No. 5,201,949. Differences in aggregate size were achieved by flue design as described by Gonzalez in U.S. Pat. No. 4,937,064 and by Gonzalez and Musick in U.S. Pat. No. 5,538,708 and by grinding as described by Ott and Sullivan in U.S. Pat. No. 5,356,470 and by Ott, et al., in U.S. Pat. No. 5,653,793.

A dilute aqueous slurry having a TiO₂ concentration of 20 ppm was prepared for each of the sample pigments. The water used for dilution contained 0.016 g/l of (tetrapotassium pyrophosphate) TKPP as a dispersant. Total transmission for each of the samples was measured using a 1-cm path length cell on a Hunter Ultrascan spectrophotometer (available from HunterLab, Reston, Va.) which is equipped with an integrating sphere to provide analysis of total transmittance. Measurements were recorded at wavelengths of 550 and 700 nm.

Optical density for each sample was calculated using equation 1. Scattering efficiency was calculated using equation 2. Optical densities using sample B as a standard are provided in Table 2.

TABLE 2 Relative Scattering Relative Efficiency (Relative Optical S_(TiO2) in Pigment Density/Concentration) Paper Sample @ 550 nm @ 700 nm Coating A 0.89 0.80 0.79 B 1 1 1 C 1.18 0.99 0.99 D 1.18 1.10 1.08 E 1.22 1.15 1.14

As can be seen from the data in Table 2, and in FIG. 1, the relative scattering efficiency (optical density/concentration), based on measurement at 700 nm correlates much better with the relative scattering coefficient (S) in the end-use, than relative optical density/concentration, based on measurement at 550 nm. Therefore, scattering efficiencies based on transmittance measurements at 700 nm will correlate with performance of these pigments in end-use applications.

For comparison, total transmission for each of the samples was measured without an integrating sphere to provide analysis of only regular transmittance. Measurements were recorded at wavelengths of 550 and 700 nm.

Optical density for each sample was calculated using equation 1. Scattering efficiency was calculated using equation 2. Optical densities using sample B as a standard are provided in Table 3.

TABLE 3 Relative Scattering Relative Efficiency (Relative Optical S_(TiO2) in Pigment Density/Concentration) Paper Sample @ 550 nm @ 700 nm Coating A 1.00 0.86 0.79 B 1 1 1 C 1.12 1.03 0.99 E 1.14 1.04 1.14

As can be seen from Table 3, when an integrating sphere is not used, that is, there is not a measurement of total transmittance, only regular transmittance, neither measurements at 550 nm nor 700 nm provide data which correlates with performance. Performance is based on Relative S_(TiO2) as determined by the method of Abrams, et al. The results based on regular transmittance only with a measurement at 550 nm over-predict the relative performance of pigments A and C. The results based on regular transmittance only with a measurement at 700 nm over-predict performance of pigment A and under-predict performance of pigments C and E. There is no wavelength which provides a 1:1 correlation of Relative Scattering Efficiency (Relative Optical Density/Concentration) with relative S_(Ti02).

Undertones calculated according to equation 3 are provided in Table 4. Also included in Table 4 for comparison are CBU values, as determined by the method of Schaumann.

TABLE 4 Pigment Sample Undertone CBU A 0.86 15 B 0.47 12 C 1.18 18 D 0.74 14 E 0.69 14

As can be seen from Table 4, the undertone values determined using the method of this invention correlate with CBU values determined in accordance with the method of Schaumann.

Example 2

Industrial scale trials were performed at a series of paper mills to test rutile pigment B and rutile pigment D, which are described above. The trial included mills producing coated freesheet, coated groundwood, and coated paperboard. Coated freesheet papers are made using pulp fibers that have been chemically treated to remove lignin. Coated groundwood papers use 10% or more of mechanical pulps. The term “lightweight coated” (LWC) is often used interchangeably with coated groundwood. The grade structure and manufacture of these papers are described in “Pulp and Paper 1997 North American Fact Book”, Miller Freeman, Inc., San Francisco (1996), pages 183-195. The coated paperboard trials included both coated unbleached kraft (“Pulp and Paper 1997 North American Fact Book”, pages 297-301.) and coated recycle board (“Pulp and Paper 1997 North American Fact Book”, pages 303-314.).

TiO₂ scattering efficiencies were determined using measurements of coat weight, TiO₂ content, opacity and reflectance in the green region of the visible spectrum. The TiO₂ scattering coefficients were calculated by the method of McGinnis in TAPPI 1990 Coating Conference Proceedings, TAPPI Press, Atlanta, p. 23.

Light scattering efficiencies of the TiO₂ in coated paper samples were compared as pigment B was replaced with pigment D. The predicted scattering efficiency improvement achieved by replacing pigment B with pigment D based on the test method of this invention was 10%. (In Table 2, pigment B was used as a standard for comparison and assigned a relative scattering efficiency of 1, when transmittance was measured at 700 nm. Pigment D had a relative scattering efficiency of 1.10, when transmittance was measured at 700 nm.) Scattering efficiency was also determined by the method of Abrams, et al. Based on this method, the predicted scattering improvement achieved by replacing pigment B with pigment D was 8%.

Tables 5 to 7 provide data for scattering efficiency improvement based on mill performance.

TABLE 5 Coated Freesheet Scattering Efficiency Mills Improvement 1 8-12% 2 7% 3 4% 4 10%  Average 8%

TABLE 6 Coated Groundwood Scattering Efficiency Mills Improvement 5 8% 6 7% 7 7% Average 7%

TABLE 7 Coated Paperboard Scattering Efficiency Mills Improvement  8 8%  9 8% 10 7% 11 9% 12 12%  13 11%  14 5% Average 9%

As can be seen from Tables 5 to 7, there is an improvement of 7-9% for the mill trials, which supports the test method of this invention as a quick and simple for predicting light scattering efficiency. It should also be noted that using a conventional wavelength in the green region for the optical density determination (550 nm, see Table 2), the improvement in scattering efficiency is over-predicted at 18%.

Example 3

The following example illustrates the use of the present method to determine the characteristics of a paper coating formulation.

A series of paper coating compositions were prepared as shown in Table 8.

TABLE 8 Kaolin Acrylic Coating Coating ID TiO2 CaCO3 Clay Latex OD/C Undertone 1  0 100   0 16  605 0.99 2 15 50 35 14 1452 0.60 3 20 40 40 14 1702 0.49 4 25 40 35 14 1892 0.40 5 15  0 85 14 1512 0.93 6 25  0 75 14 2177 0.74 7 15 50 35 18 1469 0.65 8 25 40 35 18 1935 0.45 9 15  0 85 18 1486 0.95 10  25  0 75 18 2108 0.77

In Table 8, the Sample # is the number of the particular sample prepared having the composition shown in the row associated with the sample. The composition is shown in parts of each component per 100 parts of pigment.

The kaolin clay used was a typical #1 kaolin clay in slurry (70% by weight clay); the ground calcium carbonate slurry (70% by weight calcium carbonate) had a 0.6 micron median particle size with 9% by weight finer than 0.2 micron; the titanium dioxide was a typical paper grade slurry rutile titanium dioxide (71% by weight titanium dioxide); and the binder was a 50% by weight aqueous dispersion of n-butyl acrylate/acrylonitrile/styrene copolymer with 0.1 micron average particle size and glass transition temperature of 11° C. The formulations also included 0.35 parts of an emulsion polymer thickener.

Generally samples were prepared by weighing the desired amount of clay slurry into a pot equipped with a paddle mixer. With the mixing blade turning, the calcium carbonate slurry was added to the clay slurry. This combination was mixed for about 10 minutes to ensure the materials were evenly mixed before the addition of the titanium dioxide slurry. On addition of the desired amount of the titanium dioxide slurry, the resulting mixture was again mixed for about 10 minutes before the addition of the latex binder and the thickener. After all components were added, the composition was mixed for about 10 minutes. The composition was checked for percent solids. Water was added to achieve the desired solids level. The pH of all formulations was 8.6. In the case where clay or calcium carbonate was not present in the sample, this general preparation was followed except that the step for addition of the excluded material was skipped.

The optical density of samples for each composition in Table 8 were determined using a 150 ppm concentration of the respective sample in deionized water. The optical density was measured for the samples at 700 nm. The undertone determination was made at wavelengths of 500-700 nm. Data from these samples is presented in Table 8.

FIG. 3 shows the data in Table 8 plotted as follows: calcium carbonate formulation, low binder (samples 1,2,3,4); calcium carbonate, formulation, high binder (samples 7,8); clay formulation, low binder (samples 5,6); and clay formulation, high binder (samples 9,10).

The plot in FIG. 3 shows the shifts that occurred as the composition was changed. For the CaCO₃ containing formulation at low binder, (Curve 1), the OD/C increased, and the Undertone decreased as the TiO₂ loading was increased from 0 to 15, 20 to 25 parts.

The latex binder has relatively low optical density, but a high undertone. Curve 2 compared to Curve 1 shows that as the concentration of binder increased, the undertone increased.

The kaolin clay pigment used in this formulation has a much higher undertone than the CaCO₃ pigment. Therefore, replacing the CaCO₃ with the clay pigment in either high or low binder formulations (Curves 4 and 3, respectively), causes a large undertone shift.

If the manufacturing process in the coating kitchen is in good control for coatings containing these exact same components (Samples 2,3, and 4), every batch of a given formulation would fall on the same point of this “map” or “fingerprint” of Curve 1. Detection of shifts away from the position on Curve 1 can be used to trouble-shoot inaccurate component additions or cross-contamination with other formulations, or a change in composition in the system recycle loops. The direction of the shift can help define which components are changing.

Example 4

The method of the present invention may be used to test dispersion stability. For example, some TiO₂ slurries are sensitive to flocculation by Ca⁺² ions that are present from hard water or from systems where the TiO₂ is used in combination with a CaCO₃ slurry. Sensitivity of TiO₂ slurries to Ca⁺² may be compared by comparing the measured optical densities of slurry samples made by diluting the slurry with water solution having a known CaCl₂ concentration.

In these experiment reported below in Table 9, the TiO₂ slurry samples were first diluted to 1.5% solids in a solution of either 0.7 meq/l or 1.3 meq/l Ca⁺². After 30 minutes, the slurry was then further diluted to 20 ppm using the same concentration CaCl₂ solution as in the first dilution. The optical density was measured on these 20 ppm slurry samples. The results are shown in Table 9.

In Table 9, relative optical densities for typical commercial rutile and anatase slurries, samples A to G, are shown as the initial relative optical density measured with no Ca²⁺ present. That is, the optical density measured in a dilution to 20 ppm with water alone. The second and third data columns of Table 9 show the effect as the % loss of relative optical density when the slurry samples are made up in solutions containing Ca⁺² ion in concentrations of 0.7 and 1.3 meq/l, respectively. Samples B, D and E are sensitive to Ca⁺² as evidenced by the approximately 72 to 75% loss in relative optical density measured at a concentration of only 0.7 meq/l.

TABLE 9 *Relative % Loss at % Loss at SAMPLE Optical Density 0.7 meq/l 1.3 meq/l A Rutile 1.10 1 17 B Rutile 1.05 72  —** C Rutile 0.99 2 20 D Anatase 0.80 73  —** E Anatase 0.80 75  —** F Anatase 0.80 4 45 G Anatase 0.92 18 41 *Relative optical density is the measured optical density divided by the optical density of a standard. The optical density of the standard in this study was 8640. The standard was a commercial paper grade rutile titanium dioxide. **Since the % loss of OD for these samples was so great at the lower concentration of calcium ion, no measurement was made using 1.3 meq/l. 

What is claimed is:
 1. A method to determine light scattering efficiency of a mixture containing at least one light scattering component, comprising: (a) preparing a dilute slurry of the mixture in a continuous liquid medium having a known concentration, where concentration is the concentration of mixture solids in the slurry; (b) measuring the total transmittance (T) of the slurry at a wavelength (λ) of at least 600 nm; (c) converting the transmittance measurement (T) to optical density (OD) by the equation OD=−log (T);  and (d) dividing the optical density by the concentration of mixture solids to determine light scattering efficiency of the mixture.
 2. The method of claim 1 wherein the continuous medium is selected from the group consisting of water, organic solvents, and paint resins.
 3. The method of claim 2 wherein the continuous medium is water.
 4. The method of claim 1 wherein the light scattering component is selected from the group consisting of white pigment, plastic pigment, latex polymer particles, clay and mixtures of these.
 5. The method of claim 4 where white pigment is selected from the group consisting of rutile titanium dioxide, anatase titanium dioxide, zinc oxide, silica, calcium carbonate, calcium sulfate and clay, and mixtures thereof.
 6. A method to determine undertone of a mixture containing at least one light scattering component, comprising: (a) preparing a dilute slurry of the mixture in a continuous medium having a known concentration (C), where concentration is the concentration of the mixture solids in the slurry; (b) measuring total transmittance (T₁) of the slurry at a wavelength, λ₁, between 500 and 650 nm; (c) measuring total transmittance (T₂) of the slurry at a wavelength, λ₂, between 650 and 1000 nm; (d) converting the transmittance measurements to optical densities (OD) by the equation OD=−log (T) (e) dividing the optical densities by the concentration (C) of mixture solids in the slurry to calculate OD/C; (f) determining the undertone of the mixture by the degree of change in OD/C between λ₁ and λ₂.
 7. The method of claim 6 wherein the degree of change in OD/C is calculated according to the equation: ${{undertone} = {- \frac{\left\lbrack {\ln \frac{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}}{{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right\rbrack}{\left\lbrack {\ln \frac{\lambda_{1}}{\lambda_{2}}} \right\rbrack}}},$

wherein OD₁/C at λ₁ is the optical density calculated from the transmittance measurement at wavelength λ₁, divided by the concentration of mixture solids in the slurry and OD₂/C at λ₂ is the optical density calculated from the transmittance measurement at wavelength λ₂, divided by the concentration of the mixture solids in the slurry.
 8. The method of claim 6 wherein the degree of change in OD/C is calculated according to the equation: ${{undertone} = {- \frac{\left( {{{{OD}_{1}/C}\quad {at}\quad \lambda_{1}} - {{{OD}_{2}/C}\quad {at}\quad \lambda_{2}}} \right)}{\lambda_{1} - \lambda_{2}}}},$

wherein OD₁/C at λ₁ is the optical density calculated from the transmittance measurement at wavelength λ₁, divided by the concentration of mixture solids in the slurry and OD₂/C at λ₂ is the optical density calculated from the transmittance measurement at wavelength λ₂, divided by the concentration of the mixture solids in the slurry.
 9. A method for characterizing a mixture containing at least one light scattering component comprising comparing the of light scattering efficiency of the mixture calculated according to Equations 1 and 2 herein to the undertone of the mixture calculated according to Equation 3 or 4 herein. 